Multi-electron redox reactions, such as those catalyzed by sulfite reductases (SiRs), are poorly understood, yet important in many biological processes. In addition, Mycobacterium tuberculosis utilizes sulfite reductase in assimilation of sulfur and is therefore a potential target for therapy. SiRs electronically couple a siroheme with a [4Fe-4S] cluster to form a redox-active catalytic center. It has been proposed that the six- electron reduction of sulfite to sulfide occurs via sequential, two-electron steps, however little is known about the precise order and timing of electron and proton transfers that occur during catalysis, or how this complex process is carried out without the formation of side products. Assimilatory (aSiRs) and dissimilatory (dSiR) type SiRs are quite diverse in their structure and cofactor arrangement. The monomeric aSiR from Mycobacterium tuberculosis (mtSiR) contains an unusual covalently-linked Cys-Tyr adjacent to the active site. The multimeric dSiR from Archaeoglobus fulgidus (afSiR) contains a second pair of siroheme-[4Fe-4S] cofactors, unavailable for substrate binding. Investigating the catalytic mechanism of mtSiR and afSiR, in parallel with the prototypical multimeric aSiR from Escherichia coli (ecSiR), will advance our understanding of multi-electron catalysis. In addition, a better understanding of the catalytic mechanism of M. tuberculosis sulfite reductase could lead to the development of targeted therapeutics, and could therefore have a profound impact on global health. This training program will require the utilization of many techniques common to biochemistry and bioinorganic chemistry such as Electron Paramagnetic Resonance, Uv-vis spectroscopy, and protein expression and purification. Hence, it will prepare the participant for a successful career as a biochemist. In addition, the training program offers training in the highly specialized area of protein electrochemistry, offering training in techniques such as Protein Film Voltammetry and potentiometry. Upon completion of this program, the participant will be well prepared to enter the workforce as a highly skilled scientist. PUBLIC HEALTH RELEVANCE: Multi-electron redox reactions, such as those catalyzed by sulfite reductases, are a poorly understood, yet important in many biological processes (1). In addition, Mycobacterium tuberculosis utilizes sulfite reductase in assimilation of sulfur during latency (2,3). A better understanding of these processes could lead to treatment of diseases related to enzymes that catalyze these complex reactions, in particular, this study could lead to novel treatments of M. tuberculosis infection.